MATTER AND ITS PHYSICAL PROPERTIES. 



37 



quently used even by authors pretending to scientific accuracy, is still more 

 reprehensible. It is a contradiction in terms : the term inactivity implying the 

 absence of all force. 



Before we close this subject, it may be advantageous to point out some 

 practical and familiar examples of the general law of inertia. The student 

 must, however, recollect that the great object of science is generalization, and 

 that his mind is to be elevated to the contemplation of the laws of nature, and 

 to receive a habit the very reverse of that which disposes us to enjoy the de- 

 scent from generals to particulars. Instances, taken from the occurrences of 

 ordinary life, may, however, be useful in verifying the general law, and in im- 

 pressing it upon the memory ; and, for this reason, we shall occasionally, in 

 the present treatise, refer to such examples : always, however, keeping them 

 in subservience to the general principles of which they are manifestations, and 

 on which the attention of the student should be fixed. 



If a carriage, a horse, or a boat, moving with speed, be suddenly retarded or 

 stopped by any cause which does not at the same time affect passengers, riders, 

 or any loose bodies which are carried, they will be precipitated in the direc- 

 tion of the motion ; because, by reason of their inertia, they persevere in the 

 motion which they share in common with that which transported them, and are 

 not deprived of that motion by the same cause. 



If a passenger leap from a carriage in rapid motion, he will fall in the di- 

 rection in which the carriage is moving at the moment his feet meet the ground ; 

 because his body, on quitting the vehicle, retains, by its inertia, the motion 

 which it had in common with it. When he reaches the ground, this motion is 

 destroyed by the resistance of the ground to the feet, but is retained in the 

 upper and heavier part of the body ; so that the same effect is produced as if 

 the feet had been tripped. * 



When a carriage is once put in motion with a determinate speed on a level 

 road, the only force necessary to sustain the motion is that which is sufficient 

 to overcome the friction of the road ; but at starting, a greater expenditure of 

 force is necessary, inasmuch as not only the friction is to be overcome, but the 

 force with which the vehicle is intended to move must be communicated to it. 

 Hence we see that horses make a much greater exertion at starting than 

 subsequently, when the carriage is in motion ; and we may also infer the 

 inexpediency of attempting to start at full speed, especially with heavy car- 

 riages. 



Coursing owes all its interest to the instinctive consciousness of the nature 

 of inertia which seems to govern the measures of the hare. The greyhound 

 is a comparatively heavy body moving at the same or greater speed in pursuit, 

 The hare doubles, that is, suddenly changes the direction of her course, and 

 turns back at an oblique angle with the direction in which she had been run- 

 ning. The greyhound, unable to resist the tendency of its body to persevere 

 in the rapid motion it had acquired, is urged forward many yards before it is 

 able to check its speed and return to the pursuit. Meanwhile the hare is gain- 

 ing ground in the other direction, so that the animals are at a very considera- 

 ble distance asunder when the pursuit is recommenced. In this way, a hare, 

 though much less fleet than a greyhound, will often escape it. 



In racing, the horses shoot far beyond the winning-post before their course 

 can be arrested. 



Remarkable effects of the inertia of matter are constantly exhibited in the 

 accidents from collision which take place on railways. In England, where 

 the speed is much greater than is customary in this country, such instances 

 are more frequent and fatal. The evenness and perfection of the roads and 

 carriages conspire with the extraordinary speed to render it difficult to stop 



